In general, injection compression molding of a plastic employs, as one of features thereof, injection of a molten resin or a resin in a fluid state into a cavity in closed dies having socket-and-spigot structures. Then, a relative movement of the dies is caused in the die closing direction by an amount corresponding to the length of the socket-and-spigot structure so as to fluidize the resin in the cavity to distribute the resin to all portions of the die cavity. The dies are then compressed and cooled, thus completing the molding.
In this type of injection compression molding machine, a gap referred to as "shear edge clearance" is formed in the socket-and-spigot structure. In order to prevent generation of burr or fin due to spreading of the resin while ensuring relief of gases and air, it is necessary that the amount of the shear edge clearance be maintained to be about 0.02 to 0.05 mm. Therefore, dies used in this type of molding machine are machined strictly in predetermined size to provide this clearance. In addition, it has been proposed and attempted to independently control the die temperature, in order to avoid non-uniform thermal expansion and contraction while providing optimum cooling rate of the resin. As shown in FIG. 6, the dies are connected to a slide 61 and a bolster 62 of the molding machine. The connection of the dies to the slide 61 and the bolster 62 are conducted through the intermediary of heat-insulating plates 63, in order to prevent transmission of heat while avoiding any undesirable change in the socket-and-spigot gap attributable to a lack of uniformity in the thermal expansion caused by non-uniform temperature distribution of the dies.
On the other hand, the structural members such as the slide, bolster and frames supporting these members also exhibit thermal expansions. The thermal expansions of these members also affect the socket-and-spigot gap. In view of this problem, it has been a common measure to maintain the temperature of the structural members at the same level as the working oil for actuating the press of the machine, so as to maintain a constant value of the shut height which is the distance between the bolster and the slide when the slide is in its lower stroke end. (Japanese Patent Examined Publication No. 62-11960) According to this method, series-type heat exchangers composed of baffles are provided on structural parts of the press of the machine, such as upright posts. In these heat exchangers, the oil of high temperature serves to raise the temperature of the upright posts by making use of wasteful heat which is generated by the actuating assembly and heat produced by friction between parts of the guide assembly, thus maintaining a constant value of the shut height.
In this known method, however, the temperatures of the frames, slide and the bolster are independently controlled to temperatures which are different from that demanded by the actuator of the actuating portion of the machine which is the main source of heat transmitted to various structural members. As a result, different structural parts are heated to different temperatures, resulting in a non-uniform thermal expansion of the whole structure. Consequently, the size of the socket-and-spigot gap is changed despite the presence of the heat-insulating plate provided for maintaining a constant size of the socket-and-spigot gap, due to the difference in the thermal expansion of the structural parts, with the results that the stability of the compression molding and, hence, the stability of quality of the molded products, are undesirably impaired.
In the case of a compression molding machine for molding an article having a large projection area, e.g., 500 mm or greater in side length, the lengths (L mm) of frame or other members made of steel well exceed 1000 mm. In such a case, it is necessary that the temperatures of these members have to be maintained below the temperatures (T.degree. C.) shown in the following Table 1 in order to maintain the amounts of thermal expansion (.DELTA.L mm) below the limit values shown in the table.
TABLE 1 ______________________________________ L mm T .degree.C. .DELTA.L mm ______________________________________ 1000 100 1.20 1000 10 0.12 1000 5 0.06 1000 1 0.01 ______________________________________
Under these circumstances, an object of the present invention is to provide an apparatus for and a method of controlling the temperature of a plastic compression molding machine, which is improved to maintain a constant size of the socket-and-spigot gap between the dies regardless of thermal expansions of the bolster, slide and frames, thus ensuring high stability of injection compression molding of a plastic while eliminating secular change.